Paintbrushes are commonly manufactured by bundling bristles of natural or synthetic origin, inserting one end of the bundle into a ferrule containing the handle and then potting the end portion of the bristle bundle within the ferrule with a curable liquid embedment compound. Rubber based adhesives were once used but epoxy-based compositions have long been the materials of choice for this purpose. The epoxy resin systems are made by a variety of suppliers and have a number of advantages, including good to excellent solvent resistance, good adhesion to the metal ferrules and the various bristle materials, such as polyester, nylon, polyolefin, and natural hair bristles. Also epoxies give excellent bond strength and are available at relatively low cost. However, epoxies have some significant disadvantages. These include very long cure times (4 to 8 hours at room temperature) which creates long manufacturing cycles because uncured epoxies have little to no strength so that brushes can't be trimmed until cured. Further, to achieve adequate penetration into the aforementioned bristles, up to eight different epoxy formulations are needed with wide blend ratios, 6:1 up to 13:1 by volume are common. Deep penetration into the bristles is necessary to achieve good bond strength.
Recently there has been reported in U.S. Pat. No. 4,525,890 a polyurethane embedment compound which provides good solvent resistance due to a high crosslink density. The polyurethane embedment compounds used in this reference are said to significantly shorten the manufacturing cycle of a paintbrush because of their much shorter pot-lives. However, a particular disadvantage of polyurethanes, especially ones that are highly crosslinked, is that mix ratios must be carefully controlled to achieve the proper adhesion and physical properties. A deviation in either direction of the polyol or isocyanate component of up to five percent can have a significant detrimental effect on the adhesive's final properties.
A need therefore remains for a rapid curing alternative paintbrush embedment compound which has good solvent resistance and adhesion to bristle materials and which is not highly sensitive to mix ratio.
Polyisocyanurates are known to have good chemical resistance. However, formulations curing to polyisocyanurates typically foam readily, due to production of CO.sub.2 as a byproduct of the cure reaction. Therefore, polyisocyanurates would be expected to be unsuitable as candidate paintbrush potting compounds since any foam at the bristle interface reduces adhesive bonding and reduces penetration of the compound into the bristles.